Depending on the edition of Marketo you have, you might try the Opportunity Influence Analyzer, which lets you show all the touchpoints on an opportunity, across each contact. If you have access to Revenue Cycle Explorer, there are some great Multi-Touch (MT) reports in there that can get you more of a macro view. Alternatively, if you're mapping your program memberships to Salesforce Campaigns, you can rely on Salesforce Opportunity Influence reporting, though that has some limitations.